Scientists at the LIGO Livingston Observatory in Louisiana, shown here in an
aerial shot, are searching for evidence of gravitational waves.
Photo: LIGO Laboratory
The race is on to detect ripples from the most massive events in the
universe: spinning, orbiting, exploding or colliding ultra-dense objects
like black holes and neutron stars.

In 1918, Albert Einstein predicted these cosmic events would radiate a
propagating distortion of space and time: gravitational waves. After
spending hundreds of millions of dollars to detect them, scientists have
come up empty.

But don't write off the hunt just yet. Physicists worldwide have been
fine-tuning enormous, multimillion-dollar machines to filter out background
noise so they can observe the unique signatures of a gravitation wave.
Before the decade is out, they believe they'll record the percussive crash
of colliding black holes or the vibrant hum of a pulsar -- a discovery that
would be the proverbial shot heard around the scientific world.

"I tell students they're lucky," said Rana Adhikari, a principal
investigator at the Caltech-MIT Laser Interferometer Gravitational-Wave
Observatory. "They're getting in at the right time -- it's right before we
see something."

The first concrete proof that gravitational waves exist will not only verify
a key tenet of relativity theory, but provide unprecedented insight into the
mysterious lives of black holes, neutron stars, quark stars (if these
controversial objects exist), cosmic strings (also controversial) and
probably other as-yet unimagined treasures.

Scientists have spent more than a generation tinkering patiently, coming up
empty again and again, but in the process creating increasingly powerful
tools.

The DIY set has even gotten into the act. A scientist at the University of
Massachusetts at Dartmouth has strung together eight Sony PlayStation 3s to
form a supercomputer powering a search for gravitational waves.

Other groups on the hunt have let loose much bigger machines. Stefano Foffa
of the University of Geneva is a member of a leading
gravitational-wave-detection team, which includes 33 other scientists from
Switzerland and Italy. They recently submitted a report to Classical and
Quantum Gravity that details their so-far fruitless attempts at observing
tiny gravitational tugs and distortions on Explorer, a supercooled,
3-meter-long aluminum bar at the CERN particle physics lab in Switzerland.

Explorer is particularly well-tuned to sense spinning neutron stars, also
known as pulsars, Foffa said. He and his colleagues estimate that some
200,000 of these spinning, super-dense objects -- so dense that a just sugar
cube-sized amount weighs as much as the entire human race -- are scattered
throughout the Milky Way.

But the thermal noise of even supercooled atoms is greater than the
momentary twang the bar's atoms would experience when being plucked by a
passing gravitational wave. So the Explorer group must use sensitive
superconducting circuits to coax out a signal. It's an art that's still
being perfected.

LIGO, the Caltech-MIT observatory, is an even bigger and more ambitious
project than Explorer. To someone flying overhead, LIGO looks like an
unfinished oil pipeline, with two mile-and-a-half long tubes jutting in
perpendicular directions from a central building. The pipes (one in
Livingston, Louisiana, and the other in >Richmond, Washington), contain
sensitive optics in which laser light bounces back and forth 100 times, then
combines, allowing physicists to compare the two beams to monitor the
space-time through which the light traveled.

The interference patterns from LIGO's two perpendicular laser beams
sometimes momentarily jostle. If the same jostling happens at both LIGO's
Louisiana and Washington detectors, and no earthquakes can explain the
anomaly, then the source may well be a gravitational wave.

It's the million-dollar moment that hasn't happened.

Then again, LIGO has produced mountains of data since it first began
operating in 2002. One popular distributed computing project, Einstein@Home,
sifts through these databases to check for signals that might have been
missed.

Before last year, however, the echoes of a black-hole collision were too
shrouded in complicated mathematics for scientists to even begin hunting
for. But in 2006 three separate teams cracked the numerical code to
calculate the gravitational crashing sound that merging black holes would
make.

And now LIGO scientists have begun searching their data for this
gravitational wave signature. If scientists continue to detect nothing,
however, Einstein's theories may well need modifying.

"If we don't see anything in four years," Foffa said, "then it will be the
time to start questioning."